This invention relates generally to outdoor industrial storage tanks. In particular, the invention relates to a method for installing a suspended ceiling in a storage tank such as those commonly used to store liquid materials at temperatures above or below ambient temperature. For example, liquefied gases such as methane, ethylene, propylene, propane, butane, and ammonia are commonly stored at temperatures below ambient temperature in flat-bottomed insulated storage tanks that operate at near atmospheric pressure. Such a storage tank must have adequate insulation on the roof of the tank, as well as on its bottom and shell.
Installing and maintaining insulation on the bottom and shell of a tank generally present no special problems. The tank bottom is generally flat and the shell is generally cylindrical with only one radius of curvature. Panels of insulation can be easily applied to such surfaces. Alternatively, Larsen, U.S. Pat. No. 3,991,842, describes a movable, vacuum-secured apparatus that can apply polyurethane insulating foam on the exterior surface of a shell and simultaneously cover the exterior surface with a metallic vapor barrier. As another alternative, Wissmiller, U.S. Pat. No. 3,147,878, describes a shell insulation that incorporates a second, outer shell that is spaced apart from the inner tank shell, establishing an insulating space between the two shells. A resilient blanket consisting, for example, of a material such as glass fibers is applied to the exterior surface of the inner shell while a free-flowing, lightweight, thermal insulation material consisting of a material such as expanded perlite is placed between the resilient blanket and the outer shell.
The more complicated shape of the roofs of most industrial storage tanks makes installation of roof insulation more difficult. The roof of such a storage tank may be conical, spherical, or ellipsoidal. Pre-assembled insulation panels for such shapes are difficult to fabricate and install. Because of these difficulties, special alternative insulating techniques have been developed. One common technique is to spray polyurethane foam directly on the exterior surface of the tank roof. Bellafiore et al., U.S. Pat. No. 4,333,973, describes one apparatus and method for spraying polymeric foamed-in-place insulation on horizontal and sloped surfaces such as tank roofs.
In addition to the installation problems, the exposure of roof insulation to ambient environmental conditions makes maintenance of roof insulation more troublesome. Rainwater and water vapor degrade the insulating effectiveness of externally-applied roof insulations. To delay such degradation, roof insulation for storage tanks is conventionally provided with a vapor barrier. However, the vapor barrier is exposed to the daily and annual changes in ambient temperature and solar insolation, as well as to rain, wind, ice, and snow, and generally deteriorates after a period of years. Deterioration of the vapor barrier can also result from thermal expansion and contraction of the tank, from personnel and equipment moving over the insulation surface, and from animal activity, all leading to degradation of the roof insulation.
Often, it is not possible or cost-effective to repair degraded roof insulation, and the insulation is instead entirely removed and replaced. This often requires that the tank be taken out of service for a period of time while the old insulation is removed and replaced.
Since the effective life of externally-applied tank roof insulation is relatively short, it has been found to be desirable to look for other types of roof insulation for insulated storage tanks.
Sattelberg et al., U.S. Pat. No. 3,352,443, describes an insulating ceiling that can be suspended from the inside surface of the roof of a storage tank. Because the metal tank roof protects the insulation carried by the suspended ceiling, such internal suspended ceilings have a significantly longer working life than externally-applied roof insulations. As a result, there would be a benefit in converting existing insulated storage tanks with externally-applied roof insulations into tanks with insulation supported by an internal suspended ceiling.
The Sattelberg et al. method of installing a suspended ceiling in an existing storage tank requires direct access to the underside of the tank roof in order to fasten the ceiling to the internal structural framework inside the tank roof. Unfortunately, since the roof is often 50 or more feet above the ground, this is not always easy or convenient, and externally-applied roof insulation remains in common use.